Many commercially important polymers, such as polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl acetate), and copolymers such as acrylonitrile-butadiene-styrene, styrene-butylmethacrylate, styrene-divinyl benzene, styrene-diethylaminoethyl methacrylate, and copolymers of vinyl chloride with acrylic or methacrylic acid esters are produced by suspension polymerization. In this process, droplets of monomer, or monomer mixture, containing dissolved catalyst are dispersed in water and each droplet polymerizes independently to form a discrete particle of polymer.
As the suspension polymerization proceeds, the monomer droplets progress from the liquid state through a viscous, sticky, polymer-monomer mixture to solid, spherical polymer particles. A major problem in this process is to prevent coalescence of the particles as the product passes through the viscous, sticky, polymer-monomer mixture stage. This can be accomplished by the use of certain stabilizers. Materials which have been used heretofore as suspension polymerization stabilizers have mostly been water-soluble polymers such as gelatin, pectin, starch, methyl cellulose, CMC, hydroxyethyl cellulose, acrylic or methacrylic acid polymers and copolymers, poly(vinyl alcohol) and poly(vinyl pyrrolidone); or inorganic powders such as kaolin, talc, magnesium silicate, magnesium carbonate, barium sulfate, aluminum hydroxide, tricalcium phosphate, and calcium oxalate.
The use of currently known stabilizers results in some emulsion polymer being formed simultaneously with formation of suspension polymer. An excess of emulsion polymer formation can lead to fouling of the reactor and, of course, reduces the yield of the desired suspension polymer. Salts, such as sodium sulfate, can be added to reduce this by-product, as described in U.S. Pat. Nos. 3,183,208 (Jurgeleit) and 3,642,733 (Anderson et al). See, also, U.S. Pat. No. 3,205,204 (Heckmaier).
While most of the conventional stabilizers work well as stabilizers, they are not totally satisfactory because the particle size of the resultant polymer beads, as well as particle size distribution, i.e., polydispersity, is not easily controlled. Control over polydispersity is especially important in manufacture of products, such as disposable coffee cups and packaging materials, where final density and porosity are determined by initial polydispersity. To control the particle size and polydispersity the stabilizer is usually used in combination with small amounts of a strong surface active agent such as a wetting agent or a soap of the type employed in well known emulsion polymerization procedures.
Landoll, in U.S. Pat. No. 4,352,916, teaches use of hydrophobically modified water-soluble, nonionic cellulose ether polymers, such as hydrophobically modified hydroxyethyl cellulose (HMHEC), hydrophobically modified hydroxypropyl cellulose (HMHPC) and hydrophobically modified methyl cellulose (HMMC), as stabilizers for suspension polymerization of vinyl monomers, such as styrene, etc. These hydrophobically modified water-soluble, nonionic cellulose ether polymers have been found to be useful in controlling the size of the resultant polymer particles and in reducing the amount of emulsion polymer (latex) formed as a by-product of the suspension polymerization process. However, there is still a desire to control suspension polymer particle size distribution, i.e., polydispersity, and to produce a suspension of greater clarity (reduced cloudiness or haziness due to emulsion polymer by-product adsorption onto suspension polymer beads) than is obtained by the suspension polymerization process disclosed by Landoll since, among other things, polymer size distribution control is important in manufacture and clearer suspensions are aesthetically pleasing.